The present study investigated the effect of crude extracts from yacon leaves, obtained by aqueous cold (YC) or hot (YH) extraction, or hydro-ethanolic extraction (YE), on glycemia of the diabetic (induced by streptozotocine, 50 mg/kg, ip) and nondiabetic rats. The animals were treated orally with the extracts (400 mg/kg) or water for 3, 7, 10, and 14 days. After this period, the animals were anesthetized and blood samples were collected for measurement of plasmatic glucose and creatinine and activity of the enzymes (AST, ALT, ALP). Also were evaluated the consumption of food, water, urine volume, body weight of the animals and LD50 of YE. Treatment with YE for 14 days reduced glycemia in diabetic and nondiabetic animals. No similar reduction was observed in animals treated with YC or YH. Additionally, YE restored the activity of the plasmatic enzymes that were altered, and improved weight gain in the diabetic animals. These results showed that the effectiveness of the yacon extracts is related to the method of preparation and to the time of treatment. The effect of YE on glycemia in diabetic rats is not related to reduction of food intake or to interference of the extract with intestinal absorption of carbohydrates.